What are the advantages of precharge resistors for precharging the DC link capacitor of electric cars compared to DC-DC converters?

Why is it necessary to precharge the DC link capacitor?

The heart of the power electronics is the DC link. This contains high-capacity capacitors to stabilize the power supply to the electric motor.

• The problem: When the high-voltage battery is switched on, a discharged capacitor acts like an electrical short circuit (“inrush current”).

• The consequences: Without precharging, the massive current surge would immediately weld the contacts of the main relays (contactors). This thermal and mechanical stress reduces the service life of the DC link capacitors. In addition, other components such as shunts and fuses can also be overloaded.

• The solution: Every precharge circuit—whether using a precharge resistor or a DC-DC converter—must limit the current flow until the voltage in the capacitor is equal to that of the battery.

The classic precharge resistor including precharge relay

A passive component that is briefly connected to the circuit when the electric car is started.

• How it works: When starting, the current flows in a controlled manner through the precharge resistor, limiting the inrush current and charging the DC link capacitor. As soon as the voltage difference is low enough, the resistor is bypassed.

The DC-DC converter

An active, software- and control-dependent solution that can also precharge the DC link from a technical perspective.

• How it works: Since modern electric cars already have a DC-DC converter to charge the 12V battery from the high-voltage battery, this process can theoretically be reversed.

Less complexity = fewer errors

A DC-DC converter is a complex system consisting of power semiconductors, coils, and control logic.

The risk: The more active components involved in a critical process such as vehicle start-up, the higher the statistical probability of failure.
The Miba advantage: As a passive component, the precharge resistor does not require complex control. This simplicity makes it extremely resistant to electronic interference.

Independence from the 12V charge status

This is a crucial point for everyday reliability:

• If you use the DC-DC converter in your electric car to precharge the DC link capacitor, you are dependent on a stable 12V electrical system. If the 12V battery is weak, the converter cannot start up the intermediate circuit and the car will not start.
• The precharge resistor uses energy directly from the high-voltage battery. The condition of the 12V battery is irrelevant. This guarantees a safe start-up process for the electric car, even if the low-voltage system is weak.

No load due to continuous operation

This is one of the biggest differences in the life cycle of the components:

• The DC-DC converter must continuously supply the 12V vehicle electrical system (lights, infotainment, control units). It is under constant thermal stress.
• The precharge resistor is only loaded for a few milliseconds per driving cycle. The rest of the time it is inactive and therefore subject to virtually no wear-related aging process.
   

Why is the precharge resistor for precharging the DC link capacitor now the more future-proof technology?

There is a clear trend in automotive development: reducing weight and complexity by eliminating the classic 12V lead-acid battery.

Consequence: If manufacturers dispense with heavy batteries in the future, all systems that previously relied on this energy source (such as DC-DC-based precharging) will have to be redesigned at great expense.
Consistency: The precharge resistor, on the other hand, is a physical constant, regardless of how the vehicle electrical system architecture (400V, 800V, or without 12V) develops. This makes it the safer investment for next-generation platforms.

In summary, while DC-DC converters can theoretically handle precharging in electric cars, the robustness, independence from the vehicle electrical system, and future-proofing clearly favor the precharge resistor for precharging the DC link capacitor.